Soon-Mok Choi

High-temperature charge transport and thermoelectric properties of a degenerately Al-doped ZnO nanocomposite

2 mol% Al-doped ZnO nanoparticles were consolidated into a ZnO nanocomposite with ZnAl2O4 nanoprecipitates by spark plasma sintering and its high-temperature charge transport and thermoelectric properties were investigated up to 1073 K. The carrier concentration in the nanocomposite was not dependent on the temperature, while the Hall mobility showed positive temperature-dependence due to grain boundary scattering. The negative Seebeck coefficient of the nanocomposite was linearly proportional to the temperature, and the density of the state effective mass (md*) was evaluated to be 0.33me by using the Pisarenko relation. Drastic reduction of thermal conductivity (k < 2 W m−1 K−1) was achieved in the nanocomposite, and the maximum ZT of 0.34 was obtained at 1073 K.

An enhancement of a thermoelectric power factor in a Ga-doped ZnO system: A chemical compression by enlarged Ga solubility

Herein, we report a significant enhancement of the thermoelectric power factor in polycrystalline Ga-doped ZnO. Despite its higher carrier concentration, the Seebeck coefficient of Zn0.985Ga0.015O was larger than that of Zn0.990Ga0.010O benefiting from an enhancement of the density of states (DOS) effective mass. A gradual increase in the compressive stress with Ga substitution gave rise to a higher DOS at the bottom of the conduction band. An enlarged solution limit of Ga in the ZnO matrix due to a lower firing temperature accelerated the chemical compression. A single phase n-type Zn0.985Ga0.015O bulk exhibited a power factor of 12.5 μWcm−1 K−2.

Thermoelectric properties of Cu-dispersed bi0.5sb1.5te3

A novel and simple approach was used to disperse Cu nanoparticles uniformly in the Bi0.5Sb1.5Te3 matrix, and the thermoelectric properties were evaluated for the Cu-dispersed Bi0.5Sb1.5Te3. Polycrystalline Bi0.5Sb1.5Te3 powder prepared by encapsulated melting and grinding was dry-mixed with Cu(OAc)2 powder. After Cu(OAc)2 decomposition, the Cu-dispersed Bi0.5Sb1.5Te3 was hot-pressed. Cu nanoparticles were well-dispersed in the Bi0.5Sb1.5Te3 matrix and acted as effective phonon scattering centers. The electrical conductivity increased systematically with increasing level of Cu nanoparticle dispersion. All specimens had a positive Seebeck coefficient, which confirmed that the electrical charge was transported mainly by holes. The thermoelectric figure of merit was enhanced remarkably over a wide temperature range of 323-523 K.PACS: 72.15.Jf: 72.20.Pa

Boundary Engineering for the Thermoelectric Performance of Bulk Alloys Based on Bismuth Telluride

Thermoelectrics, which transports heat for refrigeration or converts heat into electricity directly, is a key technology for renewable energy harvesting and solid-state refrigeration. Despite its importance, the widespread use of thermoelectric devices is constrained because of the low efficiency of thermoelectric bulk alloys. However, boundary engineering has been demonstrated as one of the most effective ways to enhance the thermoelectric performance of conventional thermoelectric materials such as Bi2 Te3 , PbTe, and SiGe alloys because their thermal and electronic transport properties can be manipulated separately by this approach. We review our recent progress on the enhancement of the thermoelectric figure of merit through boundary engineering together with the processing technologies for boundary engineering developed most recently using Bi2 Te3 -based bulk alloys. A brief discussion of the principles and current status of boundary-engineered bulk alloys for the enhancement of the thermoelectric figure of merit is presented. We focus mainly on (1) the reduction of the thermal conductivity by grain boundary engineering and (2) the reduction of thermal conductivity without deterioration of the electrical conductivity by phase boundary engineering. We also discuss the next potential approach using two boundary engineering strategies for a breakthrough in the area of bulk thermoelectric alloys.

Nanograined thermoelectric Bi2Te2.7Se0.3 with ultralow phonon transport prepared from chemically exfoliated nanoplatelets

Herein, we report on a scalable synthesis of surfactant-free Bi2Te2.7Se0.3 nanocrystals by chemical exfoliation and subsequent spark plasma sintering to fabricate nanostructured thermoelectric bulk materials. The exfoliated n-type Bi2Te2.7Se0.3 nanoplatelets were shown to transform into nanoscroll-type crystals (∼5 nm in diameter, ∼50 nm in length) by ultrasonication. The thermoelectric performance of the Bi2Te2.7Se0.3 nanocrystals was found to be recoverable by minimizing surface oxides by chemical reduction of the exfoliated suspensions. Nanostructured bulk materials, composed of plate-like grains with ∼50 nm thickness, were prepared by sintering of the ultrasonicated sample using a spark plasma sintering technique. The resulting compound showed drastic reduction of lattice thermal conductivity (0.31 W m−1 K−1 @ 400 K) due to enhanced phonon scattering at highly dense grain boundaries without deterioration of the power factor (21.0 × 10−4 W m−1 K−2 @ 400 K). The peak ZT value of the present compound (∼0.8 @400 K) is comparable to that of n-type single crystalline Bi2(Te,Se)3, which is one of the highest among the reported values for n-type materials synthesized by a soft chemical route.

Influence of Silicon Doping on the Properties of Sputtered Ge2Sb2Te5 Thin Film

Ge2Sb2Te5 is a promising candidate material for next-generation memory devices due to its fast phase-changing characteristics. The power consumption of an electronic device is a key factor in determining the working efficiency of next-generation memory devices. In this study, Si was doped on Ge2Sb2Te5 thin film to reduce the power consumption during device operation. Both the crystal fraction and the electrical properties of the Si-doped Ge2Sb2Te5 film were measured using in situ measurements. The analyses of the crystal structure by X-ray diffraction indicated that the doped Si and the Ge2Sb2Te5 formed a solid solution, where the doped Si occupied the interstitial sites of the Ge2Sb2Te5 crystal. Si doping was shown to elevate the crystallization temperature and enhance the thermal stability of Ge2Sb2Te5.

Enhanced thermoelectric properties of Au nanodot-included Bi2Te3 nanotube composites

Herein, we report on a scalable synthesis of Au nanodot (Au-ND)/Bi2Te3 nanotube (BT-NT) nanocomposites by the bottom-up synthesis of hybrid raw materials and subsequent spark plasma sintering, and their thermoelectric properties were systematically compared with those of Au-doped Bi2Te3 compounds. The Au nanodots were included as seeds and co-crystallized in the crystal growth of BT-NTs, which were well-dispersed in the Bi2Te3 matrix as nanoinclusions (10–20 nm). The thermoelectric performance (ZT) of the Au-ND/BT-NT nanocomposite was found to be enhanced by ∼67%, compared to pristine Bi2Te3 due to electron energy filtering and phonon scattering effects in the presence of embedded Au-NDs. The resulting compound showed an enhanced power factor (23.0 × 10−4 W m−1 K−2 @ 440 K, 27% improvement) and a reduced lattice thermal conductivity (0.47 W m−1 K−1 @ 440 K, 22% reduction). The peak ZT value of the present compound (0.95 @ 480 K) is larger than that of n-type single crystalline Bi2(Te,Se)3, which is one of the highest among the reported values for n-type Bi2Te3-based materials synthesized using a soft chemical route.

Enhanced thermoelectric performance of n-type Cu0.008Bi2Te2.7Se0.3 by band engineering

We herein report the significantly improved thermoelectric performance of n-type Bi2Te2.7Se0.3 polycrystalline bulks through band structure engineering achieved by Au-doping. The Seebeck coefficient can be increased for both Bi2Te2.7Se0.3 and Cu-intercalated Bi2Te2.7Se0.3 bulks by doping Au on the Bi site, either due to the addition of the resonant state or the enhancement of density of states (DOS) effective mass md*. Theoretical calculations combined with experimental measurements showed that band engineering connected with chemical potential tuning results in higher DOS at the bottom of the conduction band and increases the md* from ∼0.88m0 (Bi2Te2.7Se0.3) to ∼1.06m0 (Cu0.008Bi1.99Au0.01Te2.7Se0.3). As a consequence, a peak thermoelectric figure of merit ZT ∼0.91 was obtained at 320 K for Cu0.008Bi1.99Au0.01Te2.7Se0.3, which is ∼40% and ∼25% enhancement in comparison with Bi2Te2.7Se0.3 and Cu0.008Bi2Te2.7Se0.3, respectively.

An optimization of composition ratio among triple-filled atoms in In 0.3-x-y Ba x Ce y Co 4 Sb 12 system

Bulk nanostructured materials are important as energy materials. Among thermoelectric materials, the skutterudite system of CoSb3 is a representative material of bulk nanostructured materials. Filling a skutterudite structure with atoms that have different localized frequencies (also known as triple filling) was reported to be effective for lowering thermal conductivity. Among studies representing superior power factors, In-filled skutterudite systems showed higher Seebeck coefficients. This study sought to optimize the composition ratio among the triple-filled atoms in an In0.3-x-yBaxCeyCo4Sb12 system. The composition dependence of the thermoelectric properties was investigated for specimens with different ratios among the three kinds of filler atoms in the In0.3-x-yBaxCeyCo4Sb12 system. In addition, the process variables were carefully optimized for filled skutterudite systems to obtain a maximum ZT value.

Highly transparent and conducting graphene-embedded ZnO films with enhanced photoluminescence fabricated by aerosol synthesis.

Graphene/inorganic hybrid structures have attracted increasing attention in research aimed at producing advanced optoelectronic devices and sensors. Herein, we report on aerosol synthesis of new graphene-embedded zinc oxide (ZnO) films with higher optical transparency (>80% at visible wavelengths), improved electrical conductivity (>2 orders of magnitude, ∼ 20 kΩ/□), and enhanced photoluminescence (∼ 3 times), as compared to bare ZnO film. The ZnO/graphene composite films, in which reduced graphene oxide nanoplatelets (∼ 4 nm thick) are embedded in nanograined ZnO (∼ 50 nm in grain size), were fabricated from colloidal suspensions of graphene oxide with an aqueous zinc precursor. These new luminescent ZnO/graphene composites, with high optical transparency and improved electrical conductivity, are promising materials for use in optoelectronic devices.